The present invention relates to a microcomputer controlled electrostatic copying machine comprising improved means for ensuring accurate feeding of a copy sheet into toner image transferring engagement with a photoconductive drum.
Prior to the introduction of microcomputer technology, electrostatic copying machines were controlled by wired logic circuits which became quite complicated and large sized where used in high-speed, multi-mode copying machines. The circuitry became especially cumbersome where sensors were provided to monitor the ambient temperature, humidity and the like and optimally control the copying machine in response thereto in addition to controlling the basic sequence of operation.
Recently, microcomputer technology has developed to such an extent that a microcomputer comprising a central processing unit (CPU), random access memory (RAM), read-only memory (ROM) and input-output interface (I/O) can be embodied by one or more large scale integration (LSI) integrated circuit chips at quite low cost. Such microcomputers have been adapted to control electrostatic copying machines and enable elimination of a large amount of the electronic circuitry previously required.
A problem has remained heretofore unsolved in such microcomputer controlled copying machines in that the timing of feeding a copy sheet into toner image transferring engagement with a photoconductive drum has remained heretofore inaccurate. This results in the toner images being transferred out of register onto the copy sheets.
Typically, such a copying machine comprises a pulse generator which is driven synchronizingly by rotation of the drum. The pulse generator may be embodied by a disc rotatably driven by the drum formed with a series of circumferentially spaced slots therethrough. A light source and photosensor are mounted on axially opposite sides of the disc. Rotation of the disc causes the slots to alternatingly uncover the photosensor which produces electrical pulses in response thereto. The pulses are counted by the microcomputer for controlling the sequence of operation of the copying machine by means of incrementing a program counter.
A prior art method of timing the initiation or actuation of a feed roller for feeding the copy sheet into engagement with the drum comprises the step of sensing a predetermined position of a member of an optical system which scans an original document and focusses a light image thereof onto the drum. When said position is sensed, a sensor produces an electrical signal. A counter is provided to count the pulses produced by the pulse generator. In response to the signal, the counter is reset. Further rotation of the drum causes more pulses to be generated which are counted by the counter. When the count in the counter reaches a predetermined value, the feed means for the copy sheet is actuated for operation. The predetermined number corresponds to the length of time required for the toner image on the drum to reach the proper image transferring position after production of the signal.
Although this method is attractive in theory, it is inaccurate in actual operation. In order to ensure effective control of the copying machine, the pulses are generated at relatively large time intervals such as 10 ms (frequency of 100 Hz). As the scanning operation is not synchronized with the pulse generation, the sensor signal may be generated at any time between the leading edges of the pulses. This enables a maximum timing error of 10 ms to occur in the feeding of the copy sheet and allows a maximum register misalignment of 10.sup.-2 V to be produced, where V is the surface speed of the drum.
In addition to the above problem, the prior art method is not applicable to a copying machine in which an original document is fed relative to a fixed optical system for scanning. The relative register error is multiplied by the reduction ratio where reduced size copies are produced.